A thin NCD film, which has grain sizes of nm dimensions, represents a low surface roughness, low deposition temperature, low residual stress, and equiaxed grain structure, as compared to a columnar structure of a thin microcrystalline diamond (MCD) film having grain sizes of micron level. Accordingly, the NCD film has been applied to a wide variety of fields.
A microwave plasma-assisted CVD and a hot filament CVD have been most widely employed in depositing the NCD film. The microwave plasma-assisted CVD produces less contamination during process, and high decomposition efficiency due to the use of plasma, but has disadvantages of requiring an expensive equipment. Whereas, the hot filament CVD has advantages in that it is cost-effective and readily employable to mass production and to make large area films.
One of the most important interests in depositing the NCD film is to reduce the size of grains to a level of nm and also uniformly implement a microstructure thereof. This microstructure characteristic may include the size and distribution of grains, the surface roughness, the existence of pinholes or voids, and so on. Of them, the size and distribution of grains is the most important parameters. For example, there is a possibility that large grains having several hundreds nm or μm or more in size may be included in a NCD film comprised of grains having a size of a level of 20 to 50 nm. This inclusion of these abnormal large grains is a known phenomenon in microstructure of ceramic sintered body and hard carbonaceous sintered body, and there has been made many efforts to prevent such phenomenon because it causes to degrade fracture strength (see [Cha et al. Material Science and Engineering A, 356:381-389, 2003] and [Cho et al., J. Am. Ceram. Soc., 87(3):443-448, 2004]).
For example, in documents [Wei Liu, et al., Thin Solid Films, 467:4-9, 2004], Shr-Ming Huang, et al., Surface and Coatings Technology, 200:3160-3165, 2006] and X. T. Zhou, et al., Applied Physics Letters, 80:3307-3309, 2002], a method of reducing the size of grains by continuously applying a negative bias to a substrate during a deposition process has been attempted. However, this method gives damage to the surface of the diamond thin film due to the collision of ions having a large mass, and has disadvantages in that the content of non-diamond phase on the surface of the diamond thin film increases and residual stress significantly increases.
Further, in a document [Wu Nan-Chun, et al., Chin. phys. lett., 22(11):2969-2972, 2005], a method of applying a positive bias to a substrate while lowering a gas pressure to 7 torr or less in an electron-associated (EA) CVD process were used in order to make finer size of grains to a size of a nm level. This document discloses that since the fine grains are generated by the collision of electrons caused by the positive bias, a mean free path of the electrons increases by lowering the gas pressure, thereby increasing the acceleration of electrons and thus, resulting in finer grains. However, in this document, a carbon source (e.g., CH3COCH3) including oxygen was used, and the range of a gas pressure used was very low. This document also reported that a gas pressure of 15 torr generates MCD, whereas lowering of the pressure to 7.5 torr and 0.75 torr stepwise can reduce grain sizes.
As described above, although many efforts have been made to prevent the inclusion of abnormal large grains occurring in the microstructure of a NCD film deposited by a hot filament CVD process, there is still a need for the improvement of this technique.